The present invention relates to an improvement of a pulverized fuel combustion burner, provided in a boiler furnace or a chemical industrial furnace.
A conventional pulverized coal burner, as one pulverized fuel combustion burner will now be explained with reference to FIGS. 28 and 29. Reference numeral 1 denotes an air blow box, numeral 2 denotes a pulverized coal conduit provided in a central portion of the air blow box 1, numeral 3 denotes a secondary air nozzle mounted at a front end portion of the air blow box 1, and numeral 4 denotes a flame maintaining plate mounted at a front end portion of the pulverized coal conduit 2. A passage (for the pulverized coal plus primary air) is formed within the pulverized coal conduit, and a passage (for secondary air) is formed between the air blow box 1 and the secondary air nozzle 3, and the pulverized coal conduit 2 and the flame maintaining plate 4.
In the pulverized coal burner shown in FIGS. 28 and 29, the combustion is maintained by secondary air after the self-flaming of the pulverized coal fed from the burner to the pulverized coal conduit 2 by a radiation heat of the environment and a circulated eddy of the primary air formed on an inner surface of the flame maintaining plate 4.
The conventional pulverized coal burner shown in FIGS. 28 and 29 suffers from the following problems. First of all, in order to maintain a stable ignition of the pulverized coal, it is necessary to keep an A/C (primary air amount/pulverized coal amount) of the internal surface of the flame maintaining plate 4 in s range less than 2 to 2.5. However, as the combustion load is reduced, the A/C is increased (*1), resulting in an unstable ignition and increase of NOx (*2).
*1: In order to maintain the pulverized coal delivery flow rate and in view of the practical use of the pulverizing mill, it is impossible to decrease the primary air amount below a predetermined level. PA1 *2: In a certain range of the air ratio, there is a tendency that, as the higher the air ratio of the ignition portion, the more NOx becomes generated in a main burner region. The farther the ignition point, the higher the air ratio becomes, due to the diffusion of the secondary air. Accordingly, the NOx generation will become high.
Also, the pulverized coal fed from the burner into the pulverized coal conduit 2 is subjected to the self-framing effect by the radiation heat of the environment and the recirculation eddy of the primary air formed on the internal surface of the flame maintaining plate 4. The metal temperature of the flame maintaining plate 4 is kept at a high level so that clinker is liable to be stuck to the inner surface of the flame maintaining plate 4.
The clinker grows on the inner surface of the flame maintaining plate 4 toward the outer edge portion, and finally projects from the secondary air blow outlet. It becomes a factor degrading the diffusion of the secondary air and preventing the effective combustion.
Also, in the conventional pulverized fuel combustion burner, a pulverized coal concentration distribution has not been imparted between the burner conduit central portion and the vicinity of the inner wall of the burner passage.
An example of another conventional pulverized coal burner is shown in FIGS. 30 and 31. It includes a pulverized coal delivery conduit 01, a pulverized coal mixture 02, a distributor 03, a burner 04, a pulverized coal conduit 2, a concentrated burner 06, a weak burner 07, secondary air 08, air blow box 1 and a secondary air nozzle 3.
The burner 04 is formed by integrally forming the concentrated burner 06 having a high concentration of the pulverized coal and the weak burner 07 (having a low concentration of the pulverized coal in use). Each of the concentrated burner 06 and the weak burner 07 is composed of the pulverized coal conduit 2 disposed in the central portion thereof, the air blow box 1 surrounding its periphery, a rectangular pulverized coal nozzle 2a in communication with an outlet portion and the second air nozzle 3. The pulverized coal 02 that has been delivered through the pulverized coal delivery conduit 01 together with the primary air is distributed and fed to the concentrated burner 06 and the weak burner 07 by the distributor 03, respectively, and is injected into the furnace through the pulverized coal conduits 2 and the pulverized coal nozzles 2a. Thereafter, the pulverized coal is mixed and diffused with the secondary air 08 injected through the secondary air nozzles 3.
FIG. 32 is a graph showing a relationship between the air ratio and the generated NOx amount in combustion of the pulverized coal. In FIG. 32, a "volatile stoichiometric air amount" means the stoichiometric combustion air amount at which the volatile component contained in the coal may complete the combustion, and a "coal stoichiometric air amount" means the stoichiometric combustion air amount at which the coal itself may complete the combustion. As is apparent from FIG. 32, the NOx generation amount is reduced on both sides of the primary air/coal ratio of 3 to 4 (kg/kg coal) as a peak. In the pulverized coal burner, the pulverized coal mixture 02 is divided into a high concentration mixture and a low concentration mixture by the distributor 03, is introduced into the concentrated burner 06 and the weak burner 07, respectively, and is burnt at point C.sub.1 and point C.sub.2 (point C.sub.0 in total), respectively, to thereby suppress the generation of NOx and to stabilize the combustion.
Also, with respect to the pulverized coal burner to be applied to an actual system, a plurality of sets of burners each constructed as described above are assembled in the vertical direction into a one-piece type system continuous in the height direction of the furnace. Namely, as shown in FIG. 33, the duct and the burner blow box for the combustion air to be fed to the pulverized coal flame are of the one-piece type in a continuous form in the vertical direction. Also, the pulverized coal conduit for supplying the mixture of the pulverized coal and the air to the furnace is branched into a plurality of pipes having different concentrations in pulverized coal and the mixture is thus injected into the furnace.
The conventional pulverized coal burner suffers from the following problems. Since the duct and the air blow box for the combustion air to be supplied to the pulverized fuel flame is of the vertically continuous one-piece type, the overall height of the larger one reaches ten and several meters. Then, since the air blow box is mounted on boiler tubes, a thermal stress is generated due to a difference in elongation between the boiler tubes, kept at a high temperature, and the air blow box, kept at a low temperature. There is a tendency that the higher the height of the air blow box, the larger the difference in elongation and the thermal stress will become. Accordingly, in the conventional burner, there is a fear that an excessive elongation difference or thermal stress would be generated.
Furthermore, since it is impossible to provide a structure for supporting the furnace (i.e., back stays) midway of the one-piece type blow box, it is necessary to provide excessive support structures at upper and lower portions of the air blow box, resulting in a disadvantageous increase of cost.
Since the atomizing fuel supply conduit for supplying the mixture of the pulverized fuel and the air into the furnace is branched into a plurality of passages by the distributor, the structure becomes complicated, and the large number of the pulverized fuel outlets are provided, which leads to the factor of further increasing the height of the air blow box.
Also, furthermore, the conventional pulverized coal burner suffers from the following problems. In order to reduce the NOx generation amount and to stabilize the ignition, it is most preferable to use a combination of the concentrated burners 06 and the weak burners 07 for attaining the rich and lean fuel distribution. However, for this reason, the height of the panel of the burners is increased, the durable service life is shortened, and the overall structure of the burners 04 is complicated by the increase of the number of dampers.
The structure of the distributor 03 for adjusting the rich and lean pulverized coal mixture 02 becomes complicated.
For those reasons, the manufacture, control, maintenance and the like are very troublesome, which leads of a factor to increasing the cost.